Searching for Extraterrestrial Artifacts

Note: This web version is derived from an earlier draft of the paper
and may possibly differ in some substantial aspects from the final published
paper.

The Fermi Paradox, attributed to a famous question from physicist Enrico Fermi
in 1943, asks: if there are intelligent beings elsewhere then, in time, they
must achieve the technology of nuclear power and space flight and would explore
and colonize the Galaxy. Thus, they should have been able to travel to Earth,
but we see no evidence of such visitations. Ergo, they cannot exist. The author,
of the Xenology Research Institute in California, discusses this viewpoint and
suggests how and where we might be able to detect an alien presence in the Solar
System.

Introduction

If we believe
in the technology, Fermi is saying, we cannot believe in extraterrestrial life.
It is a question of "Where Are They?" All discussions of the Fermi Paradox have
made one critical assumption that should be challenged: the absence of
extraterrestrials or their artifacts on Earth or in the Solar System is an
undisputed fact. Actually, the vastness of our ignorance in this area is not
generally appreciated.

The idea that aliens might have sent an interstellar probe here to
reconnoitre our star system and its environs is not implausible. We sent Viking
to Mars to search for life. The alien device would be something like our own
interstellar probes, the four Pioneer and Voyager spacecraft, but more
sophisticated.

A typical alien probe might be 1 to 10 m in size, large enough to house a microwave
antenna to report back and to- survive micrometeorite impacts for millions of
years, yet fight enough to fly across the interstellar abyss without consuming
too much energy. Where might it be? Finding it is rather like searching a beach
for one special, oddly-shaped grain of sand.

Where Have We Looked?

A spherical
Solar System boundary enclosing the orbit of Pluto consists of 260,000 cubic AU
(astronomical units, the mean Sun-Earth distance) of mostly empty interplanetary
space and 1011 km2 of planetary and asteroidal surface. To
be able to say for certain that there is no alien presence in the Solar System,
a careful search would have to be made for artifacts.

The ability of a telescope to detect faint objects is measured by its visual
magnitude limit. The unaided eye can see down to sixth magnitude. The sky is
exhaustively and repeatedly surveyed by amateurs to, at best, magnitude +14. The
Palomar Schmidt Sky Survey extends to +21, but these plates are just 'snapshots'
of small areas and cannot be counted as a search. The best telescope on Earth
reaches only to +24.

These three magnitude limits correspond roughly to an unmoving, mirror-shiny,
optimally-oriented 10 m object orbiting at 0.01, 0.25 and 1 AU from Earth,
respectively. A smaller, moving, dark or angled object would be even harder to
see. So we can scan at best only the nearest cubic AU of space for probes, but
there is 260,000 cubic AU to search. Even if Mount Palomar was employed
exclusively to look for alien artifacts it could scan only one-millionth of the
necessary volume. Orbital space, in other words, is at least 99.9999% unexplored
for 1 to 10 m objects.

Now consider probes on planetary surfaces. Of the 1011
km2 of Solar System territory outside of Earth, less than 50 million
has been examined down to 1 to 10 m resolution. So 99.95% is still virgin
territory for this purpose. If objects are buried somewhere or floating in the
Jovian atmosphere, there is no chance yet that they could have been found. Even
huge 1 to 10 km artificial alien habitats in the Asteroid Belt would be visually
indistinguishable from asteroids to terrestrial observers, and the Belt
population itself is poorly catalogued.

So it is very unlikely that an extraterrestrial artifact in the Solar System
would be seen unless it was deliberately signalling its presence.

Detecting an operating self-replicating machine system is only marginally
easier to observe. Likely sites are the Asteroid Belt and the outer Jovian and
Saturnian moons. Recent technical studies suggest that individual replicating
systems may be 100 m in diameter or less, so a factory system for building
probes should not exceed 0. 1 to 1 km in size, again well beyond our vision
except on the Moon and portions of Mars. Ignition of fusion rockets to propel
daughter probes out of the Solar System could be spotted using amateur
equipment, but the observation window is very small and of very short duration.
Self-reproducing probes should be able to replicate a whole generation in 1000
years or less, and be quickly on their way, so that only mining pits and small
debris may remain at this late date.

The total mass of probes needed to explore the Galaxy is actually very small.
A study by the author showed that a self-reproducing probe, patterned after the
Daedalus starship design but capable of entering orbit at its destination, could
have a fully-fuelled mass of about 1010 kg. If such a device makes 10
replicas during each of 11 generations, that is enough to examine every star in
the Galaxy. This takes 1011 x 1010 kg = 1021
kg, or about the mass of Ceres, the largest asteroid. How would we ever know if
one Ceres-size asteroid had once been consumed?

More likely, starfarers would require each target star system to supply no
more than one new generation of replicants. This is only 1011 kg,
enough to fill one 1 km crater 40 m deep or to make one 400 m-wide asteroid.

Even more likely, ETI (Extraterrestrial Intelligence) would programme their
automata to erect self-replicating probe factories only in uninhabitable star
systems and send only non-reproducing exploratory probes here to avoid
disturbing any possible inhabitants.

The BIS Daedalus probe is an early attempt at spreading Man's
presence throught the Galaxy. (Painting by Don Dixon, donated by Space
Frontiers Ltd)

The Artifact Hypothesis

Combing
the entire Solar System seems a daunting task, yet radioastronomers remain
enthusiastic about the search for extraterrestrial intelligence (SETI because
the seemingly infinite "search space" of the total electromagnetic spectrum can
be narrowed considerably by imposing a set of reasonable assumptions on the
search. Many quests for interstellar radio beacons and signals have been
proposed and actually conducted in recent decades, based on three assumptions:

that advanced ETI exist in the Universe,

that these intelligences are presently attempting to locate, examine or
possibly communicate with us;

there are unique "magic" frequencies for inter stellar communication (e.g.
the waterhole).

Is it possible to similarly constrain the search
space for nearby interstellar probes and artifacts? Start with a fundamental
assumption, the 'Artifact Hypothesis':
A technologically advanced extraterrestrial civilization
has undertaken a long-term programme of interstellar exploration via
transmission of material artifacts.Now, if the Artifact Hypothesis
is correct and unless the programme has only just begun, some evidence of this
extraterrestrial exploratory activity should be apparent within the Solar
System. If persistent observation gives no support to the Hypothesis, then the
Hypothesis and possibly some of the other most basic assumptions in SETI should
be questioned.

Of course, the nature of observable artifacts depends in part upon unknown
alien motives for sending them: artifacts not intended for discovery will not be
found. For instance, in one scenario the probe imperfectly camouflages itself to
test our technology or intelligence level, which test must be passed before
communication with the device is permitted. Since the alien engineers are
technologically superior, we would not stand a chance of finding their artifact
until we meet their (unknown) conditions. But there are infinitely many possible
such "test" conditions. This leads to two conclusions. First, a search for
objects not intended to be found is probably fruitless. Second, a failure to
find alien objects not intended to be found cannot support or refute the
Artifact Hypothesis, so all such observations are a waste.

Similarly, there is no point in looking for artifacts that want to be found -
they would already be obvious.

That only leaves artifacts with a "neutral" attitude - that do not care about
outsiders. The search for "neutral" artifacts provides an excellent
observational opportunity because the search can be confined to those places
where probes are most likely to be stationed solely for scientific expediency.
Our search can therefore be defined in terms of objective criteria: the site
would have been chosen strictly for reasons of efficiency, maintainability, high
research payoff and low environmental risk.

This painting by David Hardy for the novelette 'Saturn Alia' by
Grant Callin in the July 1984 issue of "Analog" deals with the discovery of an
alien artifact on the surface of the Saturnian moon Enceladus.

Where To Look?

An active alien
exploratory probe capable of self-repair would stake its claim to the best
possible location for monitoring phenomena relevant to its mission of seeking
out life and intelligent species. To maximise the efficient use of resources,
alien engineers would have designed it to be as simple as possible yet still be
successful. Artifacts must likely satisfy two critera:

Criterion I. Ability to consistently monitor
environments most likely to harbour or evolve intelligent life.

Criterion II. Maximum artifact lifespan with
minimum complexity.

The only place that life has existed for aeons, it appears, is the Earth, clearly
the most exotic place in this Solar System. Earth is the principal target for
continuous surveillance and a superbly-crafted intelligent probe must be expected
to understand this. Criterion I thus requires the artifact
to be sited either in orbit near Earth or the Moon, or in an orbit that frequently
carries it close enough to Earth to permit adequate periodic surveillance. Terrestrial
surface sites are unlikely because these would restrict the ability of the probe
to continuously monitor the entire environment. Even if the main probe were
not situated near Earth it would likely deploy permanent surveillance subprobes
in the vicinity which would themselves be detectable. In the lexicon of radio-SETI,
circumterrestrial space is a "waterhole" region where extraterrestrial probes
may be expected to congregate, seeking life and intelligence.

Criterion II (maximum lifespan)
implies that it would attempt to spend as much time as possible in regions of
low environmental hazard. It will seek out locales with minimum high-energy
particle intensities and electric and magnetic field densities and minimum danger
from micrometeorite and debris impacts. This rules out planetary magnetospheres
and ring systems.

Also, to maximise lifespan, the artifact must have access to sufficient energy.
Self-contained systems are unlikely to provide enough power for data processing,
self-repair operations, orbit/attitude control and interstellar radio transmission.
An onboard fusion power plant is possible, but most likely the artifact will
collect solar energy. This requirement, plus Criterion I, eliminates all outer
planet sites.

Finally, to minimise probe complexity, orbits must be dynamically stable over
long periods of time - this eliminates most heliocentric orbits. Criterion II
also argues strongly against siting the artifact on the surface of any
celestrial body having

an appreciable escape velocity requiring a major propulsion system for
deorbit or ascent,